Question

When warmed in dilute sulfuric acid, 1-phenyl-1,2-propanediol undergoes dehydration and rearrangement to give 2-phenylpropanal. Its constitutional isomer, 1-phenyl-1-propanone, is not formed under these conditions. Following are the mechanistic steps leading to the formation of this unobserved isomer: Add a proton to give protonated alcohol 1; Break a bond to give carbocation intermediate 2; 1,2-shift to give carbocation intermediate 3; Carbocation intermediate 3 is stabilized by resonance delocalization with carbonyl 4; Loss of a proton leads to the formation of 1-phenyl-1-propanone. Write out the mechanism on a separate sheet of paper, and then draw the structure of carbocation intermediate 3 in the box below. You do not have to consider stereochemistry. You do not have to explicitly draw H atoms. Do not include lone pairs in your answer. They will not be considered in the grading.

Answer 1

The mechanism is as suggested

1) Protonation of alcohol

2) removal of water molecule to give carbocation

3) hydride shift to give more stable carbocation [rearrangement]

4) the stability of carbocation due to resonance

5) removal of proton and formation 1-phenyl-1-propanone.

This is Pinacole pinacolone rearrangement

Thus the reaction can be depicted as